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Author Topic:   Human Chromosome 2 and the Evolution of Humans
Wounded King
Member (Idle past 1539 days)
Posts: 4149
From: Edinburgh, Scotland
Joined: 04-09-2003


(1)
Message 16 of 56 (668291)
07-19-2012 12:00 PM
Reply to: Message 14 by herebedragons
07-19-2012 11:29 AM


just because we don't have an explanation right now doesn't mean there isn't one.

That is rarely if ever the answer given, we frequently don't have a complete answer and an answer of sufficient depth to satisfy the ID proponents and creationists seems to be a logical impossibility, but there is usually a plausible explanation which is swiftly discounted by the ID/creation side. You seem to be similarly using 'plausible' to mean' complete and detailed in every aspect'.

And we would also not expect to find the first or only person who had such a rearrangement, but a representative of the population.

So is your question, "If an incredibly statistically unlikely thing happened might it lead us to make incorrect conclusions?", If it is then I'm not sure why you even had to ask.

That aside, if the genetic material was sufficiently well preserved that they could derive a karyotype from it there would be enough material to show that they were virtually identical genetically, so they might well classify it as a sister species or a case of incipient speciation.

Chromosome 2 is presented as evidence of human - chimp evolution (and I agree it's quite a strong piece of evidence) but if it is not a plausible explanation for the evolution of chimp - human, doesn't that still leave us without a plausible explanation?

You are conflating two distinct things here, the common ancestry of humans and chimps, for which Human Chromosome 2 is held up as a strong piece of evidence, and the general divergence between the chimp and human lineages and how it has given rise to the distinct modern species, in which the chromosome 2 fusion is only really of interest as one of a number of differences, and is not a particularly interesting one compared to many as it has been linked to no clear functional consequences.

In studies of hybrid infertility/speciation in Drosophila they have identified dozens of individual mutations each of which is sufficient to establish a strong barrier to reproduction between populations, but they can't all be the initial causative basis for reduced reproduction between the ancestral populations when they first split. Similarly there are almost certainly dozens of genetic factors which would prevent humans and chimps viably interbreeding including the differences in karyotype, all of these may constitute plausible explanations but that doesn't let us identify the chromosome 2 fusion or any of the others as the specific causative basis of the initial reproductive isolation of the ancestral populations.

TTFN,

WK

Edited by Wounded King, : No reason given.


This message is a reply to:
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PaulK
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Joined: 01-10-2003
Member Rating: 3.0


Message 17 of 56 (668305)
07-19-2012 2:18 PM
Reply to: Message 14 by herebedragons
07-19-2012 11:29 AM


quote:

That's kinda what I thought you meant, which is why I asked for clarification. Because that's not really true is it? Isn't that what IDers and creationists do with irreducible complexity? Since there is no plausible explanation as to how this could have evolved then it must be intelligently designed. And the answer is usually "just because we don't have an explanation right now doesn't mean there isn't one." So lack of a plausible explanation may cause us to question the hypothesis, but is not a means of falsification.

You need to be careful there. That is the position some ID supporters retreat to, since the actual IC argument has failed but even there they demand a huge amount of detail, which I do not.

The key issue is the level of knowledge we have. Obviously we do not have the knowledge to reconstruct the complete history of a complex adaption including all the relevant mutations, in order and the selective pressures that caused them to become fixed in the population. By my understanding we do have a fairly good idea of the sorts of mutations that are possible, and if we could not explain the chromosomal difference in those terms we would have a problem. If we did NOT have that knowledge, it would be a different matter.

quote:

Why would we not if we had limited other clues to go on. Differing chromosome numbers usually indicates infertility. In the case of the 44 chromosome man, we have many other clues that he is still of the human race. With the limited amount of ancestral human remains we have, any significant differences would give reason to separate them into different species. And we would also not expect to find the first or only person who had such a rearrangement, but a representative of the population.

Of course you are assuming infertility, but it would seem a huge leap to conclude a new species without morphological differences. While cryptic species do exist I think that we would want a little more. And don't forget that the article on the 44 chromosome man says he would have fewer infertility problems than his parents, even if his partner had 46 chromosomes.

quote:

No, of course it wouldn't happen immediately. Is there a more plausible explanation for the break between humans and chimps? Chromosome 2 is presented as evidence of human - chimp evolution (and I agree it's quite a strong piece of evidence) but if it is not a plausible explanation for the evolution of chimp - human, doesn't that still leave us without a plausible explanation?

Well it isn't a plausible explanation for the whole evolution of humans and chimps, at most it is a component of how the populations that evolved into humans and chimps diverged. The very fact that the new chromosomal arrangement was able to spread into the population and persist argues that the infertility problems were not too severe, at least at first.

However, there's a problem here. The chromosomal difference can only significantly contribute to one population split, one speciation event. But there must have been several between us and our common ancestor with the chimpanzees. How do you narrow it down to the split between our ancestors and those of chimpanzees rather than one of the later speciation events ?


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Dr Adequate
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Message 18 of 56 (668324)
07-19-2012 8:42 PM
Reply to: Message 13 by herebedragons
07-19-2012 10:19 AM


Yes, but in this case we are talking about a specific mutation. [...] But IMHO, in order to be conclusive there needs to be a plausible explanation.

Yes, but the explanation, though plausible, is allowed to be very unlikely with respect to the specific mutation, because there must have been lots of this kind of mutation which, as it happened, didn't get fixed.

So, for example, I don't think we necessarily need to provide an explanation "without invoking another spontaneous B-T". Why not invoke another? That would help explain why this particular one got fixed and all the others didn't. Nor can we rule out other forms of "dumb luck", as you call it. It is perfectly plausible that this was the one single balanced translocation that got lucky between the common ancestor of apes and the array of modern species we have now. All the fusions of different chromosomes in different branches of the apes didn't make it. One did. That suggests that there was an element of luck, a fluke event and not a predictable one.


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herebedragons
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From: Michigan
Joined: 11-22-2009
Member Rating: 5.9


Message 19 of 56 (668363)
07-20-2012 10:12 AM
Reply to: Message 16 by Wounded King
07-19-2012 12:00 PM


That is rarely if ever the answer given,

No, you rarely give an answer like that, and there are others that take the time to explain things well. But from what I have seen, I don't think it is an unreasonable summary of the response to ID and creationists demands for "proof" of evolutionary processes. I am not being critical of the response or defending ID positions because sometimes it is just true, we don't have all the answers.

an answer of sufficient depth to satisfy the ID proponents and creationists seems to be a logical impossibility, but there is usually a plausible explanation which is swiftly discounted by the ID/creation side.

Yeah. On another forum I saw an admin demanding evidence for macro-evolution by asking for every single intermediate step between two different kinds and gaps of millions of years were not permissible. Basically, it would take a sample of every organism that ever lived to be sufficient.

You seem to be similarly using 'plausible' to mean' complete and detailed in every aspect'.

I don't mean to come across that way. I guess by plausible I mean so that I can understand it and make sense of it While the ToE makes sense to me in general, it is the particulars and the details, especially when applied to historical events, that I am still tentative about. So I often take the devil's advocate position. I am willing to learn what evolutionary biologist actually believe and why, and what the evidence tells us and make a decision from there.

That aside, if the genetic material was sufficiently well preserved that they could derive a karyotype from it there would be enough material to show that they were virtually identical genetically, so they might well classify it as a sister species or a case of incipient speciation.

Good point. I guess I tend to associate chromosome differences to indicate infertility or at least a reduction in fertility because that what we typically observe. But since it isn't necessarily the case, there would have to be more info.

all of these may constitute plausible explanations but that doesn't let us identify the chromosome 2 fusion or any of the others as the specific causative basis of the initial reproductive isolation of the ancestral populations.

Ok. makes sense. Which is why, on a phylogenetic tree, we place changes in character states in between branch points instead at branch points.

So, it brings me back to the main questions in the OP. Can we conclude that the rearrangement occurred within the human lineage? Can we then expect to find examples of Homo spp. with 48 chromosomes or would that be an unrealistic expectation?


Whoever calls me ignorant shares my own opinion. Sorrowfully and tacitly I recognize my ignorance, when I consider how much I lack of what my mind in its craving for knowledge is sighing for. But until the end of the present exile has come and terminated this our imperfection by which "we know in part," I console myself with the consideration that this belongs to our common nature. - Francesco Petrarca

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jar
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Posts: 29042
From: Texas!!
Joined: 04-20-2004
Member Rating: 2.5


Message 20 of 56 (668366)
07-20-2012 10:47 AM
Reply to: Message 19 by herebedragons
07-20-2012 10:12 AM


on chromosome changes.
Several points that might help.

In Message 7 I provided a link to a thread from about a year ago where I posted a link to an article on the "The 44 Chromosome man" that does a great job of outlining the breeding possibilities he would face. It might help you.

Second, we can extract DNA from older remains (not fossils) that go back 20,000 or maybe even 60,000 years but not from fossils. As the age of remains increases the possibilities of recovering DNA decreases and the possibility of contamination increases. It becomes increasing difficult and unlikely to get results the further back we go and if the fusion even was in early hominids as it seems, we just won't find the evidence.

Third, the evidence we do have seems to show that early hominids expanded widely and in a series of waves and so populations were scattered and isolated.


Anyone so limited that they can only spell a word one way is severely handicapped!

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foreveryoung
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Posts: 879
Joined: 12-26-2011


(1)
Message 21 of 56 (671853)
08-30-2012 10:17 PM


The thread was closed where I posted this question earlier. What would be the result if all human life was destroyed except for 7 humans? I am referring to noah and his family. What would the genetics look like after 6000 years?
Replies to this message:
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Coyote
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Posts: 5868
Joined: 01-12-2008
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(2)
Message 22 of 56 (671859)
08-30-2012 10:58 PM
Reply to: Message 21 by foreveryoung
08-30-2012 10:17 PM


What would be the result if all human life was destroyed except for 7 humans? I am referring to noah and his family. What would the genetics look like after 6000 years?

I will address mtDNA (mitochondrial DNA) as that is the only one with which I have direct experience.

You would have a severe bottleneck, with diversification from that point on expanding to fill all other areas of the world.

All mtDNA genomes from prior to a worldwide flood, commonly attributed to ca. 4,350 years ago, would be terminated -- with the sole exception of the genomes of Noah's female kin and companions.

In most areas of the world you would find skeletal populations with pre-flood mtDNA up to a certain time -- the flood -- but those would be abruptly terminated and subsequently replaced by mtDNA attributed to the Middle or Near East. Those would be Noah's female kin and companions. If there was a worldwide flood with 7 survivors the break would be very distinctive and absolute.

Instead what we find are mtDNA patterns whereby the ancient genomes persist in many cases right up through modern times.

One example: a skeleton dated >9,000 years old from southern Alaska was found to have a rare haplotype which was found in modern populations stretching from California to the tip of South America. This shows both continuity of the mtDNA haplotype and the lack of a break ca. 4,350 years ago. It also shows evidence of an early coastal migration.

A second example: from my own archaeological research I have a skeleton dated 5,300 years ago that had the same mtDNA haplogroup as modern descendants in the same area.

In both cases we have no drastic break ca. 4,350 years ago, but rather a persistence of mtDNA types from before to after the date attributed to the global flood.

These two examples are but a drop in the bucket -- there is similar, and better, evidence showing the same thing from most parts of the world.

And the evidence is not limited to genetics. We see continuity throughout the world in linguistics, human cultures, fauna and flora, as well as many other aspects of the natural world. Tree rings, glacial varves, and coral growths show no break ca. 4,350 years ago. Nor do the fields of geology or sedimentology.

While rambling, I hope this answers the question you asked.


Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.

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foreveryoung
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Posts: 879
Joined: 12-26-2011


Message 23 of 56 (671861)
08-30-2012 11:41 PM
Reply to: Message 22 by Coyote
08-30-2012 10:58 PM


I agree that there is no evidence for a GLOBAL flood 4350 years. In fact, all the evidence is against it. I am starting to believe in a mesopotamian flood around that time that destroyed the sumerian culture. I should have posed my question differently though. Would it be possible to have a thriving human population after 4350 years if it started with only 7 humans such as noah's family?
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ooh-child
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Posts: 181
Joined: 04-10-2009


Message 24 of 56 (671862)
08-30-2012 11:45 PM
Reply to: Message 23 by foreveryoung
08-30-2012 11:41 PM


How large, numerically, would you consider to be thriving?
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foreveryoung
Member
Posts: 879
Joined: 12-26-2011


Message 25 of 56 (671865)
08-31-2012 12:05 AM
Reply to: Message 24 by ooh-child
08-30-2012 11:45 PM


The current population of the world.
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ooh-child
Member
Posts: 181
Joined: 04-10-2009


Message 26 of 56 (671868)
08-31-2012 12:33 AM
Reply to: Message 25 by foreveryoung
08-31-2012 12:05 AM


Any wiggle room?
Is that your final answer, or could it be any less?
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Dr Adequate
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Joined: 07-20-2006
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Message 27 of 56 (671870)
08-31-2012 12:50 AM
Reply to: Message 23 by foreveryoung
08-30-2012 11:41 PM


I agree that there is no evidence for a GLOBAL flood 4350 years. In fact, all the evidence is against it. I am starting to believe in a mesopotamian flood around that time that destroyed the sumerian culture. I should have posed my question differently though. Would it be possible to have a thriving human population after 4350 years if it started with only 7 humans such as noah's family?

Maybe. Coyote's point (and mine on the other thread) was that the human population couldn't be as genetically diverse as it is now. Could it be as large as it is now? I don't really see why not, assuming they weren't wiped out by genetic problems associated with inbreeding. But it would not look, in detail, like the human population that we actually have.


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PaulK
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Posts: 12771
Joined: 01-10-2003
Member Rating: 3.0


Message 28 of 56 (671877)
08-31-2012 1:39 AM
Reply to: Message 23 by foreveryoung
08-30-2012 11:41 PM


I think that with an effective population size of only 5 humanity would probably go extinct. And that's without assuming a population of only 2 a couple of thousand years earlier which would make things even worse.

Aside from the genetic issues, though, I don't think that there is necessarily a barrier to the actual population size increasing hugely since then (although I think we'd still be behind current population).

That said, good as the genetic evidence is, if you put the flood 4350 years ago I don't think that the archaeological evidence shows any culture being wiped out and replaced at that time. Not even the Sumerians. If you want to avoid archaeology, you'd have to go further back in time.


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foreveryoung
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Posts: 879
Joined: 12-26-2011


Message 29 of 56 (671882)
08-31-2012 2:26 AM
Reply to: Message 28 by PaulK
08-31-2012 1:39 AM


paulk writes:

I think that with an effective population size of only 5 humanity would probably go extinct. And that's without assuming a population of only 2 a couple of thousand years earlier which would make things even worse.

Aside from the genetic issues, though, I don't think that there is necessarily a barrier to the actual population size increasing hugely since then (although I think we'd still be behind current population).

That said, good as the genetic evidence is, if you put the flood 4350 years ago I don't think that the archaeological evidence shows any culture being wiped out and replaced at that time. Not even the Sumerians. If you want to avoid archaeology, you'd have to go further back in time.

Surely you agreed that the sumerian culture disappeared at a certain point in time? There is indeed geological evidence for a substantial flood in the mesopotamian area at least in the vicinity of the time that the sumerian culture existed. What date does archaelogy give for the disappearance of the sumerian culture?


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foreveryoung
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Posts: 879
Joined: 12-26-2011


Message 30 of 56 (671883)
08-31-2012 2:38 AM
Reply to: Message 27 by Dr Adequate
08-31-2012 12:50 AM


dradequate writes:

Maybe. Coyote's point (and mine on the other thread) was that the human population couldn't be as genetically diverse as it is now. Could it be as large as it is now? I don't really see why not, assuming they weren't wiped out by genetic problems associated with inbreeding. But it would not look, in detail, like the human population that we actually have.

Inbreeding causes problems because of harmful recessive genes. If that original population of 7 from noah's family was free of harmful recessive genes, then inbreeding would not be a problem. There is a verse in genesis suggesting just that. "These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God." In my mind, perfect here refers to the physical guality of noahs body. If perfect meant "good behavior", it would be redundant since the verse already says noah was just.


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